The latest Census estimates show urban counties in the Northern U.S. and Midwest, in particular, are losing residents to the suburbs and Sun Belt.

For much of the past half century, St. Louis incurred a gradual decline as residents left the city. Like other urban areas, its population eventually stabilized with the revitalization of its central corridor and more young people moving in and around downtown. But now, the latest Census estimates suggest the slight growth may have been short-lived.

Many urban counties that had halted population declines or experienced promising gains are similarly losing residents again, according to Census estimates released Thursday.

These core urban counties, mostly found in older regions or the Midwest, are experiencing either accelerated population declines or a slowdown in growth. Areas around cities like Atlantic City, N.J.; Baltimore; and St. Louis, for example, all saw rates of population decline pick up significantly over the past year.

Governing compared the Census Bureau's updated domestic migration estimatesfor all counties within metro areas, which were then grouped by population density. The 146 most densely populated counties lost a total of 539,000 residents to other parts of the country over the 12-month period ending in July, representing the largest decline in recent years.

At the same time, many more suburban counties are now welcoming more residents. Less densely populated metro area counties experienced another uptick in domestic migration last year.

One of the main drivers of the decline for urban counties continues to be families fleeing for the suburbs.

Couples leaving the city of St. Louis often head west or across the river in search of larger homes or better school districts, says Ness Sandoval, a St. Louis University professor who studies the city’s demographic trends. The housing stock is one of the oldest of any city, so many opt for more modern homes further out rather than rehabbing vintage properties. Taking their place are not other families but often one or two single individuals who can afford to rent an entire home, according to Sandoval.

Millennials are still moving in near the university and other parts of the city, but it’s not nearly enough to offset losses from those leaving. Over the 12-month period ending in July, the city of St. Louis lost 1.1 percent of its population -- the largest decline of any large Census-designated county nationally.

The steady stream of residents leaving has led to thousands of vacant homes and lots, particularly on the north side. The city's Land Reutilization Authority currently holds approximately 12,000 vacant properties, far more than comparable cities. Meanwhile, Sandoval says, nearly all the new housing development is oriented more toward solo renters, rather than families.

“In some neighborhoods, we have condos for over $400,000, and in others, you’d be lucky if you could get $25,000 for a house,” he says.

A somewhat similar scenario is playing out in the city of Baltimore, where the population declined at the same rate. A net total of 11,000 residents moved out of the city -- many to surrounding suburban neighborhoods -- nearly doubling the domestic migration loss of the prior 12-month period.

“The suburbs in Baltimore are no more just middle-class professionals, they’re much more diverse,” says Andrew Cherlin, a Johns Hopkins University professor.

Part of why St. Louis and Baltimore registered such large declines relative to other areas is that they’re independent cities not part of larger counties, occupying smaller geographies. The Census estimates cover all counties and metropolitan areas, reflecting annual changes over the 12-month period ending in July.

Cook County, which contains Chicago, is another jurisdiction where population gains appear to have reversed. The county had reported slight population increases each year between 2010 and 2013 before losing residents over the past two years. Mounting losses from outmigration have similarly led to recent declines in Atlantic County, N.J.; Erie County, Pa.; and other larger Northern counties.

Brookings Institution Demographer William Frey notes that suburbanization dropped off sharply in most places following the mortgage meltdown. It’s now picking back up, he says, particularly across the Snow Belt and older cities.

“There’s a bit of an uptick in the exurbs and emerging suburban areas,” he says. “There could be more of it coming down the road.”

Governing also compared migration patterns using a county classification scheme defined by the National Center for Health Statistics. Average net domestic migration rates for more suburban fringe metro counties continued to climb, and rates for medium metro counties also more than doubled last year. By comparison, the central counties of large metro areas declined significantly, losing an average of 26 people for every 10,000 residents last year.

But not all residents of these predominately Northern urban areas are simply packing up and moving to the suburbs. In many cases, they’re also relocating to the Sun Belt.

Migration to the Southern U.S. slowed down dramatically during the recession and has since rebounded the past three or four years, continuing with the latest population estimates, according to Frey.

Urban jurisdictions throughout many of these Southern regions saw their numbers climb accordingly.

Consider Maricopa County, Ariz., which added more than 81,000 residents last year -- by far the most of any county in the country. Several counties throughout Florida, South Carolina and Texas also all recorded annual population increases exceeding 3 percent.

Meanwhile, industrial counties suffering losses for years generally recorded population declines that either remained steady or accelerated in the new Census release. For example, the Ohio counties of Trumbull and Mahoning, which make up the Youngstown metro area, both experienced their largest declines in recent years.

It’s also worth noting that many more urban counties, including those in the South, would be registering population declines if it weren’t for international migration. Data suggests average international migration rates for larger metro areas have changed little recently.

It’s too early to say how individual cities fared last year -- the Census Bureau is scheduled to release its 2016 city-level population estimates in May.

Domestic Migration Rates for Metro Area Counties

The following tables represent Governing calculations of average annual domestic migration rates for all counties within metro areas:

Density / Sq. Mile

2010-11

2011-12

2012-13

2013-14

2014-15

2015-16

# Counties

Less than 200

-8.3

-20.6

-4.9

1.5

2.3

21.6

643

500 to 200

15.2

23.1

24.9

32.5

32.8

40.6

268

500 to <1k

17.5

17.9

28.0

35.9

38.0

39.7

109

Greater than 1k

0.2

3.7

-3.9

-24.4

-21.4

-33.9

146

Average net domestic migration rates shown per 10,000 population. Density computed using 2016 population by square mile of land area.Separately, grouping counties using a classification scheme defined by the National Center for Health Statistics suggests similar trends. The central counties of metro areas with populations exceeding 1 million lost 26 people per 10,000 residents to other parts of the country last year.